Forms of the apparatus of the above general type are described for example in the specifications of International Patent Publication No. WO93/06423, U.S. Pat. No. 5,245,836 and U.S. Pat. No. 5,291,941. The refrigerant fluid used in these apparatuses is typically carbon dioxide (CO.sub.2).
Where it is not required to cool the air which is to be delivered into the cabin of a vehicle, but merely to heat the air, it is usual to utilize the heat which is produced by the propulsion engine of the vehicle, for example by passing air in contact with a heating radiator through which the engine coolant fluid flows. When the engine is cold, no heat can be drawn from it for the purpose of heating the cabin. This results in a considerable delay before the cabin can be brought to the required temperature, and is consequently detrimental to the comfort of the occupants of the vehicle. In some cases, the coolant liquid never, during the entire duration of a journey, reaches a high enough temperature to warm the occupants as they would wish.
In order to enable the cabin to be brought to the required temperature more quickly, various supplementary devices may be used, in particular those which involve the use of heaters or electrical radiators, but these are somewhat expensive.